sgingter
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Problem with range of 433 Mhz receiver

Sun Apr 28, 2019 12:09 pm

Hi,

I have a problem with an abysmal range of my 433 MHz receiver: If I point the transmitter exactly on the back side of the receiver, I can achieve a "range" of up to 25 cm until the receiver only shows garbage.

For what I want to achieve, I need to detect two distinct signals, from one or two transmitters (possibly sending different signals), and with a range of 25m in a ~120 degree arc in which I can reliably detect the signal, even if the transmitter is not exactly pointed at the receiver. The receiver should be placed in a small wooden box together with the Pi, a Touchscreen and a relay board (a bit of interference is expected), and the wooden box has a wall thickness of about 0.75 cm.

The worst case would be the operator is about 25m away, reaches into his trousers pocket (on the far side) to press a button and the signal then needs to bypass his butt, travel the 25m and through the wood and still be detected.

I currently use this 433 MHz receiver on my Pi 3 B+ : https://cdn-reichelt.de/documents/daten ... _DB_DE.pdf
I tested the receiver with the transmitters of this product: https://www.amazon.de/gp/product/B07K9S ... UTF8&psc=1

I don't want to spend too much on other receivers / transmitters, but if its required, then I can threw in some additional euros.

So, my question is:

Is there anything I can do to improve the reception range and detection angle of the cheap receiver I have, to achieve what I want?

Do you know other modules that will achieve the required range? When I go for other modules: 1The transmitter shouldn't be self-made. This is something another non-techy person gets to carry and press, and that should be something looking and feeling like a real thing, and not a cheap plastic case with a bunch of soldered wires around it, if you know what I mean ;)

Thanks,

Sebastian

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scruss
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Re: Problem with range of 433 Mhz receiver

Sun Apr 28, 2019 3:04 pm

Those super-regenerative receivers are really more bother than they are worth. I've never had them working reliably. RFM69W or even the superheterodyne boards work better.

Critical to all of these receivers/transmitters is antenna length. A couple of millimetres off and it'll hear nothing.

If you're just looking for a keyfob switch and don't have to be on 433.92 MHz or worry too much about security, the Simple RF M4 Receiver boards are quite easy to integrate.
‘Remember the Golden Rule of Selling: “Do not resort to violence.”’ — McGlashan.

PhatFil
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Re: Problem with range of 433 Mhz receiver

Sun Apr 28, 2019 4:11 pm

The itead sonoff rf bridge is a $12 5v powered off the shelf product which can be reprogrammed with custom firmware which can then decode and repeat via wifi/mqtt ALL 433mhz signals it can 'hear' (Most of the basic protocols are covered but not all)
https://github.com/arendst/Sonoff-Tasmo ... Bridge-433
at circa 60mmx60mmx22mm its a smallish pocketable device.. complimented by a pi AP hosting a mqtt broker and some s/w to subscribe to the relevant mqtt topic to respond to any known codes, this could be a simple python script http://www.steves-internet-guide.com/in ... on-client/ or something more sophisticated like a homeautomation suite. Might suit your requirements?

sgingter
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Re: Problem with range of 433 Mhz receiver

Mon Apr 29, 2019 8:26 am

@scruss Thanks. Sadly I can't use 315 MHz here. In Europe these frequencies are reserved for military communications, and I better not interfere with them ;)

@PhatFil Thank you. For me, that bridge feels a bit like overkill. I want the whole solution to be as easy and as reproducible as possible, even for non-techies. I will consider that if I can't get the necessary range with easier means.

I will first order the superheterodyne RXB6, RXB8 and RXB14 modules to test them out. They are just over 1€ each, and maybe they get the range I require. Also, on the RXB6 I can easily attach a wire as an antenna.

A question regarding the antenna: In several other threads I read the antenna should be a straight wire of 173mm length.
Now, where does the length begin?

From the point where it is straight, and I can have a shorter, flexible, wire as a connection to the straight "antenna part"?
Or do I have to measure the 173mm directly from the point where I solder the antenna wire on to the board?
What, if the board has a connector pin and I want to use i.e. a bread board connector wire straight up cut to length? Does the connector add up to the wire length or not?

You see, I'm a bit confused about that ;)

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mahjongg
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Re: Problem with range of 433 Mhz receiver

Mon Apr 29, 2019 8:46 am

Move the receiver far away from the RPI.

Doesn't really matter if its 433MHz, or 315MHz, both work on the same principle, of 100% on/off keying.

Signal levels are so weak, and the "modulation method" so simple, that even if 0.001% of a PI generated signal will overwhelm the receiver if its too close.
It will simply "see" the 433MHz signal all the time. A "better receiver", (meaning more sensitive) will therefore not help.

due to its nature as an electromagnetic source, a doubling of the distance from the PI to the 433MHz receiver will decrease the disturbance signal eight fold, so moving the receiver away just a meter or so will have a dramatic influence.

And yes, antennae length is very critical.

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RichardRussell
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Re: Problem with range of 433 Mhz receiver

Mon Apr 29, 2019 8:48 am

sgingter wrote:
Mon Apr 29, 2019 8:26 am
From the point where it is straight, and I can have a shorter, flexible, wire as a connection to the straight "antenna part"?
The entire length of wire will act as an antenna, but its equivalent electrical length will depend in a complicated way on whether it is straight or not. The only way you can legitimately 'extend' the connection between the PCB and the antenna is via a transmission line (i.e. typically a flexible coaxial cable) but that won't work well unless there's a good impedance match both ends, and the PCB may not be designed for that.
What, if the board has a connector pin and I want to use i.e. a bread board connector wire straight up cut to length? Does the connector add up to the wire length or not?
The connector will contribute to the 'electrical length' of the antenna, yes, but by precisely how much will again be difficult to determine. If you really want to tune the antenna for optimum performance you would need specialist test equipment.

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scruss
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Re: Problem with range of 433 Mhz receiver

Mon Apr 29, 2019 1:11 pm

sgingter wrote:
Mon Apr 29, 2019 8:26 am
@scruss Thanks. Sadly I can't use 315 MHz here. In Europe these frequencies are reserved for military communications, and I better not interfere with them ;)
True enough. Are there not local equivalents on 433 MHz of these M4 boards? They're very handy.
mahjongg wrote:
Mon Apr 29, 2019 8:46 am
Move the receiver far away from the RPI.

Signal levels are so weak, and the "modulation method" so simple, that even if 0.001% of a PI generated signal will overwhelm the receiver if its too close. It will simply "see" the 433MHz signal all the time. A "better receiver", (meaning more sensitive) will therefore not help.
Eek, I better go and remake the very successful advertising installation and the wireless power monitors I built that have the antennae pretty much in the Raspberry Pi case as they can't possibly work now …
‘Remember the Golden Rule of Selling: “Do not resort to violence.”’ — McGlashan.

MarkTF
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Re: Problem with range of 433 Mhz receiver

Mon Apr 29, 2019 3:02 pm

mahjongg wrote:
Mon Apr 29, 2019 8:46 am
Signal levels are so weak, and the "modulation method" so simple, that even if 0.001% of a PI generated signal will overwhelm the receiver if its too close.
It will simply "see" the 433MHz signal all the time. A "better receiver", (meaning more sensitive) will therefore not help.
This isn't quite right. Better receivers (e.g. RXB12), in addition to having better sensitivity, have much better selectivity. Selectivity is the receiver's ability to reject strong signals near the band of interest which is largely what one sees as unintended radiation off a digital board like the RPi.

The receiver indicated in the original post (FS1000A) is a "super regenerative" receiver and will tend to get pulled to the largest signal it sees over quite a wide band (+/- several MHz), a phenomena sometimes referred to as "receiver capture". The RXB12 is a crystal controlled "super heterodyne" receiver and will tune the desired frequency (+/- a few tens of kiloHertz) largely independent of whatever else may be going on in the environment.

MarkDH102
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Re: Problem with range of 433 Mhz receiver

Tue Apr 30, 2019 6:13 am

Signal levels are so weak, and the "modulation method" so simple, that even if 0.001% of a PI generated signal will overwhelm the receiver if its too close.
It will simply "see" the 433MHz signal all the time. A "better receiver", (meaning more sensitive) will therefore not help.
That's very interesting. I was having trouble receiving from an RFM12B transmitter (433MHz) about 75m away from my setup (Pi Zero W headless connected to an Arduino via a proprietary (mine) 2 wire (about 6cm long) interface). The Arduino handled the RFM12B receiving side of things.
So I built a replica (identical) Arduino board and connected the analogue RFM12B RSSI pin to an analogue input. I plugged it via USB into a laptop and had flawless comms with good RSSI. I was about a meter away from the Pi setup while testing with the laptop.

I will try extending my 2 wire comms and power lead to a meter in length and see what happens...

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